Define a prototype for exit in stand.h. Provide a reference to exit in
a few conf.c files to ensure that its definition gets pulled in early.
Since exit() is a MD routine, it isn't defined in libsa. However,
libsa tends to be listed last and will soon have panic() in it which
calls exit(). The reference to exit early ensures that the MD exit is
available to satisfy linking for static libraries.
Move ufsread.c from sys/boot/common (which used to be all the common
files for /boot/loader, but grew to be all the common files for
sys/boot, but that's now sys/boot/libsa's job) to sys/boot/libsa.
Sponsored by: Netflix
Build libstand from inside the sys/boot build. Redirect all users in
sys/boot to grab it from there. We still build it as libstand.a for
the moment. When lib/libstand is moved here, we'll change the name.
Sponsored by: Netflix
